There are two types of nanoemulsions, i.e., an oil-in-water (o/w) nanoemulsion and a water-in-oil (w/o) nanoemulsion. An o/w nanoemulsion has a continuous water phase and a dispersed oil phase while a w/o nanoemulsion has a continuous oil phase and a dispersed water phase.
These two types of nanoemulsions are stabilized by emulsifiers that have different hydrophilic-lipophilic balance (HLB) values. An o/w nanoemulsion is stabilized by an emulsifier having a HLB value of 8-28 and a w/o emulsion is stabilized by an emulsifier having a HLB value of 3-6. As a result, they cannot be easily inter-converted.
The lack of facile convertibility can be problematic. An o/w nanoemulsion collapses when its water content decreases. Similarly, a w/o nanoemulsion collapses when its oil content decreases.
Thus, there is a need to develop a nanoemulsion that its continuous phase and dispersed phase can be readily converted from one to the other with the same emulsifier contained in it.